1. Field of the Invention
The present invention relates to a method and apparatus for estimating the running state of a tire and the condition of a road in contact with the tire at the time of running.
2. Description of the Prior Art
To enhance the running stability of a car, it is desired that the state of a tire and the condition of a road in contact with the tire at the time of running should be estimated accurately and fed back to car control. The expression “state of a tire” means the prediction of the inside pressure, wear or trouble of a tire and the expression “the condition of a road” means mainly a friction coefficient between a road and a tire (road friction coefficient μ).
If the running state of a tire and the condition of a road can be estimated, before a trouble occurs in the tire, a car can be stopped for inspection, or before the operation of avoiding a danger such as braking or steering is made, advanced control with an ABS brake can be carried out. Thereby, the further improvement of safety is expected. Simply by informing a driver of the degree of danger of the condition of a road during driving, the driver can decelerate the car earlier. Thereby, a reduction in the number of car accidents can be expected.
In the prior art, to estimate a road friction coefficient, there are proposed a method in which a road friction coefficient is estimated making use of a phenomenon that the uniformity level of a tire which is a physical amount indicative of a change in the revolution of a wheel is changed by the size of a road friction coefficient (JP-A 2000-55790) and a method in which an accelerometer is mounted to a lower arm for interconnecting the front wheel and the car body to detect the horizontal vibration of a tire which is toed in and a road friction coefficient is estimated making use of a phenomenon that the above vibration level is changed by a road friction coefficient (JP-A 6-258196) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
However, in the above method for estimating the road friction coefficient from the uniformity level of a tire, a flat spot is formed on the tire to deteriorate the uniformity and accurate estimation is difficult in the recovery process of the uniformity.
In the method for estimating the road friction coefficient from the horizontal vibration of a front wheel which is toed in, when the slip angle of the tire becomes nil or large, the measurement accuracy is low.
There is also proposed a method of estimating a road friction coefficient from transmission characteristics between a nonsuspended acceleration which is an acceleration in the vertical direction of a wheel and a suspended acceleration which is an acceleration in the vertical direction of a car body (JP-A 11-94661). Since steering force is not used to estimate the road friction coefficient in this method, the road friction coefficient can be estimated even for a straight road where almost no steering is made. However, since the road friction coefficient is estimated from the transmission characteristics of a vibration between two points through a suspension having large buffer characteristics such as a spring or damper, it is easily affected by the unevenness of the road. For example, since a nonsuspended vibration becomes large on a rough road such as a snow road, the difference in level between a suspended vibration which is absorbed by a suspension and the above nonsuspended vibration becomes large, thereby making it impossible to estimate the road friction coefficient accurately.
It is an object of the present invention which has been made in view of the problem of the prior art to improve the running stability of a car by estimating the condition of a road in contact with tires and the running state of a tire accurately.